1. Field of the Invention
This invention relates to a method of manufacturing a monolithic integrated circuit and, more particularly, to such a circuit wherein components are formed in surface regions of a semiconductor body, which regions are electrically isolated from the remaining semiconductor body by a stepped pn-junction surface.
2. Description of the Prior Art
It is well known to produce an isolated surface region by diffusion of a frame-shaped insulating zone of a first conductivity type, through an epitaxially grown layer of a second conductivity type on a substrate of the first conductivity type. This method of isolation requires a special planar diffusion process and the associated photolithographic etching steps.
The disadvantages of the above method are avoided by a method disclosed in the German Pat. No. 1,769,271. Said German patent teaches a method wherein a highly doped surface region of opposite conductivity is diffused into a substrate prior to the formation of an epitaxially grown layer over the substrate. The epitaxial layer is of the same conductivity type as the highly doped region but has a lower doping concentration than the doping concentration of the substrate and has a controlled thickness. The thickness is controlled so that during a subsequent diffusion of an insulating zone into the epitaxial layer around the highly doped region and during the diffusion of elements of semiconductor components into the surface of the epitaxial layer above the region, the insulating zone impurities will diffuse inwardly to meet impurities diffusing outwardly from the semiconductor substrate to completely isolate a surface region of the semiconductor body with a pn-junction surface formed between the substrate type impurities and the epitaxial layer type impurities. In particular, during diffusion of the base of a planar transistor component into the region the isolating junction is completed.
In the case of monolithically integrated circuits employing a particularly low-ohmic substrate, the aforementioned method has the disadvantage that the breakdown voltage of the pn-junction between the surface region and the substrate rapidly decreases as the impurity concentration of the substrate increases. This breakdown voltage also decreases as the grown epitaxial layer becomes thinner, because the steepness of such a pn-junction increases as the Dt-product (D = diffusion constant of the dopings, t = time) decreases. A subsequent diffusion of the impurities would provide an improvement, but this causes a diminution of the sheet conductivity by diffusion of compensating impurities into the buried layer.